Rotary screw compressors having control valves to allow for escape of compressed air in a working chamber are known.
U.S. Pat. No. 3,314,597, Screw Compressor, concerns a screw compressor having a low pressure end plate and a casing. A working chamber is provided in the casing. The chamber is substantially in the form of two intersecting cylindrical bores of the same size having parallel axes located in the same horizontal plane. The casing is provided with a recess communicating with an axial high pressure port. The recess extends axially from a high pressure end wall towards the low pressure end plate. The recess and a barrel wall of the working space intersect along two straight edges parallel with the axes of the bores. The recess has a cross-section in the form of a segment of the circle. An axial slidable valve is located in the recess. The valve is provided with a cylindrical wall sealingly cooperating with the barrel wall of the recess.
U.S. Pat. No. 3,874,828, Rotary Control Valve for Screw Compressors, Herschler, et al, concerns a liquid injected helical screw gas compressor having a capacity control valve. The control valve includes a cylindrical plug fitted in a chamber which is adjacent to and in communication with the compressor's working chamber by way of a series of auxiliary ports. The control valve member includes a control edge with the auxiliary ports for controlling the gas throughput and the built-in volume ratio of the compressor.
U.S. Pat. No. 4,147,475, Control System for Helical Screw Compressor concerns a helical screw compressor having a rotary capacity control valve, a compressor inlet throttling valve, and a pressure relief valve for venting the compressor discharge conduit. A control circuit senses pressure in the compressed air supply system downstream of the compressor and operates the capacity control valve, inlet throttling valve and pressure relief valve in a predetermined sequence to provide an improved power consumption characteristic for variable demand compressor applications. The control circuit includes a hydraulic control valve and an actuator for operating the rotary capacity control valve to regulate compressor throughput to maintain a predetermined pressure without over pressuring the supply during operating conditions of reduced demand for compressed air. The compressor in part is characterized by a cylinder bore in the housing in which is disposed the rotary capacity control valve.